This invention relates to resistor devices with a high power rating and the method for manufacturing these devices.
In microelectronic assemblies, one goal is to achieve higher density circuit boards. To achieve this higher density, there is a need to reduce the size of each component. Traditionally, resistors are located near the surface of the circuit board, as depicted in FIG. 1. FIGS. 1A and 1B depict a conventional resistor device 10 of the prior art having an alumina body 12 and end terminations 14. The resistor is formed on a top surface 18 of the alumina body 12 and is in contact with end terminations 14 to form the resistor circuit. A power resistor is a resistor structured to dissipate heat. Typically, a power resistor has a resistance that is low enough to generate a significant amount of heat which can then be dissipated, for example, a resistance of 10 ohms or less. The power rating of a resistor is based on its ability to dissipate the heat generated. However, the conventional resistor device 10 must radiate heat into the air, or through the alumina body 12, neither of which is very efficient, resulting in a low power rating. Typically, to obtain a higher power rating in a conventional resistor device 10 requires a larger size device. Thus, there is a conflicting need for larger resistor devices to obtain higher power ratings and smaller resistor devices to achieve higher density circuit boards.
There is also a need to reduce part counts on the boards in order to reduce manufacturing assembly time and to reduce the number of interconnects, which can improve yields. Components referred to as xe2x80x9cintegrated passive componentsxe2x80x9d or xe2x80x9cintegrated passivesxe2x80x9d can be used to address that need. One method for producing these components is referred to as the xe2x80x9cLow Temperature Co-fired Ceramicxe2x80x9d approach, or the so-called LTCC method. The LTCC method is an outgrowth of traditional thick film ceramics, where materials are fired at around 850xc2x0 C. for about 10 minutes. None of the actual core materials are capable of sintering at these temperatures, but in the process they are mixed with a glass frit, which allows them to densify into a composite matrix having the desired properties of conductors, resistors or insulators. The goal of the LTCC approach is to take the materials traditionally used for making ceramic circuit boards, and instead use them to make complex sub-assemblies.
There is thus a need for a power resistor device of small dimension and high power rating that utilizes the benefits of the LTCC approach.
The present invention provides a power resistor device having a high power rating. To this end, the device comprises a fired ceramic chip, such as an alumina body, having internal continuous conductor electrodes or conductor plates. A resistor is formed on one or both ends of the chip, and the ends are terminated over the resistors. Because the resistor is covered with metal, which is then soldered to traces on the circuit board, better heat dissipation is achieved as compared to conventional resistors.
The present invention further provides a method for making power resistor devices in which a resistor material is applied to a first end of a fired, ceramic multi-electrode chip, such as by dipping the end in a resistor paste and firing the chip to form a resistor on the end of the chip. End terminations are then applied to both ends of the chip, such as by applying a conductor paste to the ends and firing the chip. By this method, a power resistor device is formed in which buried continuous conductor electrodes are electrically connected to the end terminations, where the connection at the first end is through the resistor material to form a resistor device structured to efficiently dissipate heat generated by the resistor. In an alternative method of the present invention, a resistor material is applied to both ends of the chip, such as by dipping both ends in the resistor paste, to form resistors on both ends of the chip. The end terminations are then formed over the resistors, such as by applying conductor paste over the resistors and firing the chip. By this alternative method, a power resistor device is formed in which the buried conductor electrodes are electrically connected to the end terminations, where the connection at both ends is through the resistor material to form a resistor device structured to efficiently dissipate heat. In yet another alternative method of the present invention, a conductor under-layer is formed under the resistor, such as by first dipping the end of the chip in a conductor paste and firing the chip. By this alternative method, a power resistor device is formed in which the buried conductor electrodes are electrically connected to the end terminations through the conductor under-layers and resistors.